Had one like that a while back. HO smelled gas from the supply registers for a couple of seconds each time the blower came on. Gas Co had been out several times and couldn't find the leak. Turned out to be a gas fireplace starter valve (inside the RA duct panning in the bsmt) leaking.
Gas would build up in the panned joist cavity when the blower was off. Then it would clear all of it out when it started.
thats weired wonder why they just did not do around the duct work. I hope you changed it lol.
either they didnt know how or was lazy.probably the 2nd one .yea we fixed it
thats where the pipe ended up on the rough in and the trim man did'nt move it when he put the unit in
sometime rough in work isnt at the best spot when you go to complete it.